Current designs for double row or larger polygonal arches present difficulties when applied to structures with spans above 40 feet (12 m) that need to meet public load safety standards, or that need to be dismantled easily and reused, or which are constructed without scaffolding, assembled without heavy equipment, and built with bamboo or other locally-available beam materials, or which need to be safely and reliably assembled by non-professionals.
What are needed are connectors that enable the construction of arch-shaped structures either individually or as parallel ribs of cylindrically-shaped structures such as supporting arches for bridges, tunnel linings, Quonset hut-type shelters and arbors. The need is for a connector that enables the construction of arches where the stringer beams are arranged in two or more parallel rows so that the ends of the beams in one row are opposite the midsection of the beams in an adjacent row. Arches constructed from straight beams are desirable because they use lower cost standard components but retain the strength, simplicity and extended span of arches constructed of specially engineered curved components.
The end-to-end alignment of beams in polygonal arches transfers the load placed on the arch to the abutments along the longitudinal axis of each beam. This end-to-end load transfer makes efficient use of the strength of most materials. Although a polygonal arch makes good use of materials, the end-to-end alignment of the beams is unstable. Adding enough bracing to make a single row of beams rigid increases costs and lowers the strength-to-weight ratio. The instability problem is solved by joining at least two parallel, end-to-end aligned rows of beams so that the point where the beams meet in one row is braced by the mid-point of a beam in the adjacent row. The resulting arch is strong, light-weight and uses readily available standard materials.
For most civil engineering projects, the trusses and curved-component arches that can be made of aluminum or steel are more efficient in their use of materials than the double row polygonal arch. However, for many remote, emergency response, environmentally-sensitive or limited-funding situations, the double row or multi-row polygonal arch would be a superior support structure for bridges and larger shelters due to its simplicity, strength and ability to span greater distances with small, human-portable components assembled by unskilled labor. To meet the requirements of these demanding situations, the structure needs to be improved so it can be built quickly and safely out of standard modules in difficult terrain, be constructed of bamboo or other local materials like small diameter timber, and be easily disassembled, transported and reused.
Various designs exist for building arches using straight beams both with and without connectors between the beams, e.g., U.S. Pat. No. 4,412,405, J. J. Tucker; U.S. Pat. No. 1,727,022, T. Ahlborn; U.S. Pat. No. 3,004,302, W. W. Nightingale; U.S. Pat. No. 3,091,002, L. E. Nicholson. Historical arch designs also provide examples, e.g., the ‘self-supporting bridge’ of Leonardo Da Vinci, bridges in rural China such as Meichong Bridge, Yunhe County, and Xidong Bridge, Taishun County, both in Zhejiang Province, and the Moon Bridge at Huntington Gardens in Pasadena, Calif. Some designs provide modularity, reusability and safety, but the benefits are limited primarily to one material, or to very small structures. A single design which addresses the combined requirements of cantilevering, allowing a wide range of beam materials, and reducing construction time, which can be scaled up to build structures with spans of 20 meters or more, is lacking.